Deep brain stimulation is a surgical treatment involving the implantation of a medical device called a brain pacemaker, which sends electrical impulses to specific parts of the brain. DBS in select brain regions has provided remarkable therapeutic benefits for otherwise treatment-resistant movement and affective disorders such as chronic pain, Parkinson's disease, tremor and dystonia
The deep brain stimulation system consists of three components: the implanted pulse generator (IPG), the lead, and the extension. The IPG is typically a battery-powered neurostimulator encased in a titanium housing, which sends electrical pulses to the brain to interfere with neural activity at the target site. The lead is typically a coiled wire insulated in polyurethane with four platinum iridium electrodes and is placed in one of three areas of the brain. The lead is connected to the IPG by the extension, an insulated wire that runs from the head, down the side of the neck, behind the ear to the IPG, which is placed subcutaneously below the clavicle or in some cases, the abdomen. The IPG can be calibrated by a neurologist, nurse or trained technician to optimize symptom suppression and control side effects.
DBS leads are located in the brain according to the type of symptoms to be addressed. For non-Parkinsonian essential tremor the lead is placed in the ventrointermedial nucleus (VIM) of the thalamus. For dystonia and symptoms associated with Parkinson's disease (rigidity, bradykinesia/akinesia and tremor), the lead may be placed in either the globus pallidus or subthalamic nucleus.
All three components are surgically implanted inside the body. Under local anesthesia, a hole about 14 mm in diameter is drilled in the skull and the electrode is inserted, with feedback from the patient for optimal placement. The installation of the IPG and lead occurs under general anesthesia.
Stereotactic procedures and specifically Deep Brain Stimulation can benefit enormously with the use of Magnetic resonance as a guidance for both more accurate placing the needles for biopsies or guiding and placing the stimulation leads for the DBS procedure. Particularly for DBS procedures, there exists a limitation for the use of magnetic resonance during and after the implantation of the electrodes on the DBS procedure. This arises due to the heating of the leads which will occur in the transmit RF field necessary for MR Imaging. The heating of the leads must be maintained at a temperature less than a specified maximum in order to avoid over heating of tissue and consequential damage.
For a DBS procedure three are in general three stages that Magnetic Resonance can be utilized as a guidance for the procedure. The first step is the placing the stereotactic frame on the patient's head that is bolted on the skull with four pins that are attached to the frame and to the skull of the patient. Then a stereotactic locating phantom is attached to the frame and the patient is ready for pre-operative imaging. The purpose of this phantom is to calibrate for any non-linearities and non-uniformities arising from the main magnetic field as well as the gradient. In addition, it provides additional information regarding image intensity correction from the B1 field of the RF transmit coil
As a next step the patient, with the stereotactic frame and stereotactic locator phantom attached to the patient, is brought to an MR table. The DBS stereotactic frame and locator phantom are placed inside a Transmit/Receive quadrature head coil and a 3D MR image is obtained that include the fiducials on the frame locator phantom. An example of a suitable head coil is commercially available from Siemens, which can be used with Medtronic's MR compatible DBS leads for 1.5 T systems only. This image is utilized by the navigation software to guide the MER (Micro electro-recordings) to the required location accurately. However, there is a drawback on this procedure. The entire DBS operation relies on the images that were obtained before the operation started. The required accuracy of placing the electrodes at the required location is within few millimetres, or the entire procedure will be unsuccessful. The issue exists, during the intra-operative stage, when a burr-hole is opened on the skull and the brain shifts by few millimetres. At that stage, using the presently available coil configuration it is impossible to place back the locator phantom on the stereotactic frame. Thus it is not possible to obtain new set of 3D MRI images to be used as a new registration platform for the navigation software. For this reason it is not possible to capture the changes in the brain movement post burr-hole and accurately guide the MER to the required location and furthermore the accurate placement of the electrodes.
Typically in imaging the patient with the DBS leads in place, the power output of the transmit coil must be limited to less than 0.1 Watts/kg utilizing the existing available head coil described above, in view of the presence of the metal of the DBS leads within the RF field and located in the brain which can cause unacceptable heating of the leads. Due to the restriction of the power that is allowed to be used when imaging DBS leads, it is impossible to obtain any good image quality at such low power levels. Some users resort to exceeding these limits and in rare instances this has resulted in the heating of the leads harming the patient or causing death.
It will be appreciated that the head coil is large and a long distance away from the head so it requires more power to generate an MRI image with an acceptable SNR .
U.S. Pat. No. 6,969,992 (Vaughan) issued Nov. 29 2005 to University of Minnesota discloses a parallel RF transceiver for an NMR system. An excitation and detection circuit has individually controllable elements for use with a multi-element RF coil. Characteristics of the driving signal, including, for example, the phase, amplitude, frequency and timing, from each element of the circuit are separately controllable using small signals. Negative feedback for the driving signal associated with each coil element is derived from a receiver coupled to that coil element.
U.S. Pat. No. 7,525,313 (Boskamp) issued Apr. 28 2009 to GE discloses a system for a multi-channel MR transmission system including transmitting multiple radio frequency (RF) channels via an RF coil assembly. An RF coil assembly having a number of coil elements is configured to transmit a number of RF channels which is less than the number of coil elements thereof. Some implementations may use signal splitters for some or all of the RF channels to produce driving inputs for each coil element. By using more coil elements than RF channels, various embodiments may exhibit increased power efficiency and improved B1 uniformity.
U.S. Pat. No. 6,982,554 (Kurpad) issued Jan. 3 2006 to GE discloses a system and method for operating transmit or transmit/receive elements in an MR system An array of series resonant transmit elements include individual control of RF current in all elements. The array adjusts scan homogeneity during a scan or prescan phase by adjusting amplitude and phase. The array also selectively excites areas of interest, thus avoiding major power dissipation and avoiding heating in the patient.
The disclosures of the above documents are incorporated herein by reference.